In order to increase the energy density and output of lithium-ion batteries, a study on using, as a negative electrode active material, a metal material that forms an alloy with lithium, such as silicon, germanium, tin, or zinc or an oxide of the foregoing metal instead of a carbon material such as graphite has been conducted.
In such a negative electrode active material composed of a metal material that forms an alloy with lithium or an oxide of the foregoing metal, lithium from a positive electrode active material is taken into the negative electrode active material during the first charge, but the lithium is not completely taken out during the discharge. An indefinite amount of lithium is immobilized in the negative electrode active material, which results in irreversible capacity. PTL 1 discloses that lithium is supplied to a negative electrode to compensate for the irreversible capacity of silicon oxide.